In the voice coding field, typical methods of coding a 50 Hz to 7 kHz wideband signal include the G722 and G722.1 standards of the ITU-T, and AMR-WB proposed by 3GPP (The 3rd Generation Partnership Project). According to these coding methods, it is possible to perform coding of wideband voice signals with bit rates of 6.6 kbit/s to 64 kbit/s. However, the quality of such a signal, although high compared with a narrowband signal, is not adequate for audio signals or when more realistic quality is required of a voice signal.
Generally, realism equivalent to FM radio can be obtained if the maximum frequency of a signal is extended to around 10 to 15 kHz, and CD quality can be obtained if the maximum frequency is extended to around 20 kHz. An audio signal coding method such as the Layer 3 method standardized by the MPEG (Moving Picture Expert Group) or the AAC (Advanced audio coding) method is generally used for coding of such wideband signals. However, with these audio coding methods the bit rate of the coding parameter is high because the frequency band subject to coding is wide.
As a technology for performing high-quality coding of a wideband signal spectrum at a low bit rate, a technology is disclosed in Patent Document 1 whereby the overall bit rate is reduced while suppressing quality degradation by replacing a high frequency band spectrum within a wideband spectrum with a duplicate of a low frequency band spectrum, and then performing envelope adjustment.
Also, in Patent Document 2 a technology is disclosed whereby the bit rate is reduced by dividing a spectrum into a plurality of subbands, calculating gain on a subband-by-subband basis and generating a gain vector, and performing vector quantization of this gain vector.    Patent Document 1: Japanese Patent Publication Laid-Open No. 2001-521648 (p. 15, FIG. 1, FIG. 2)    Patent Document 2: Japanese Patent Application Laid-Open No. HEI 5-265487